The main infection and disease-fighting cell of the human immune system is the white blood cell (leukocyte), which circulates through the blood. Approximately 50 to 65 percent of all leukocytes are a type of cell called a "neutrophil," which mediates much of the infection-fighting capability of the white cells. When a human has a substantially lower than normal number of circulating neutrophils, the patient is considered to be suffering from "neutropenia," i.e., a condition characterized by an abnormally low number of circulating neutrophils.
A patient suffering from neutropenia then is at substantial risk of infection and disease, as the diminished number of neutrophils circulating in the blood substantially impairs the ability of the patient to fight any infection or disease, as less neutrophils are available to engage in the battle. In severe cases of neutropenia there may be essentially no neutrophils available to fight infection and disease.
Neutropenia, itself, may be the result of disease, genetic disorders, drugs, toxins, and radiation as well as many therapeutic treatments, such as high dose chemotherapy (HDC) and conventional oncology therapy. For example, many cancers have been found to be sensitive to extremely high doses of radiation or anti-neoplastic (anti-cancer) drugs. These cancers include malignant melanoma, carcinomas of the stomach, ovary, and breast, small cell carcinoma of the lung, and malignant tumors of childhood (including retinoblastoma and testicular carcinoma), as well as certain brain tumors, particularly glioblastoma. However, such intensive HDC is not widely used because it frequently causes such a compromise of the hematopoietic system that the result is death due to any of numerous opportunistic infections.
The reason behind the compromise, if not devastation, of the hematopoietic system resulting from HDC is generally understood. The HDC acts upon rapidly proliferating cells in the bone marrow that produce neutrophils, platelets, erythrocytes, lymphocytes, and other leukocytes. When the hematopoietic system is functioning correctly, platelets and neutrophils proliferate rapidly and turn over at a high rate, unlike the lymphocytes and red blood cells, which are long-lived. The result of HDC, then, is that not only are cancerous (neoplastic) cells destroyed, so are the cells of the hematopoietic system that are responsible for generating the army of neutrophils that are necessary to maintain a functioning immune system. Complete destruction of neutrophil progenitor and precursor cells eliminates the patient's short-term capacity to generate mature neutrophils, thereby severely compromising the patient's ability to combat infection. The patient then becomes "immunocompromised" and subject to opportunistic infection. Such a condition may ultimately result in morbidity and death. Other situations also may be encountered where there has been a severe insult to the hematopoietic system, resulting in a substantial reduction in neutrophils and precursors thereto.
In order to understand the problems presented by neutropenia, whether caused by HDC or otherwise, it is first necessary to understand some basic principles about human blood cells, including their source and their development.
Hematopoiesis refers to the proliferation and differentiation of blood cells. The major site of hematopoiesis in humans, after about 20 weeks of fetal life, is the bone marrow. Blood cells develop from multipotent stem cells that are usually located in the bone marrow. These stem cells have the capacity to proliferate and differentiate. Proliferation maintains the stem cell population, whereas differentiation results in the formation of various types of mature blood cells that are grouped into one of three major blood cell lineages, the lymphoid, myeloid or erythroid cell lineages. The lymphoid lineage is comprised of B cells and T cells, which collectively function in antibody production and antigen detection, thereby functioning as a cellular and humoral immune system. The myeloid lineage, which is comprised of monocytes (macrophages), granulocytes (including neutrophils), and megakaryocytes, monitors the bloodstream for antigens, scavenges antigens from the bloodstream, fights off infectious agents, and produces platelets, which are involved in blood clotting. The erythroid lineage is comprised of red blood cells, which carry oxygen throughout the body.
The stem cell population constitutes only a small percentage of the total cell population in the bone marrow. The stem cells as well as committed progenitor cells destined to become neutrophils, erythrocytes, platelets, etc., may be distinguished from most other cells by the presence of the particular progenitor "marker" antigen that is present on the surface of these stem/progenitor cells. A group of antibodies that is capable of recognizing this particular marker antigen is referred to as "cluster of differentiation 34" or "CD34". The designation "CD34+" is used to describe a cell as one that has the particular cell surface antigen that is recognized by the CD34 group of antibodies. Stem cells, then, are CD34+. The majority of cells that are CD34+ in bone marrow, however, are B lymphocyte progenitor cells and myeloid progenitor cells.
Neutrophils differentiate from stem cells through a series of intermediate precursor cells, which can be distinguished by their microscopic morphological appearance, including such characteristics as the size of their nuclei, the shape of their nuclei, cell size, nuclear/cytoplasmic ration, presence/absence of granules, and staining characteristics (see FIG. 1 and Atlas of Blood Cells: Function and Pathology, second edition, Zucker-Franklin et al.). Initially, the multipotent stem cell, which cannot be measured directly in vitro, gives rise to myeloid "progenitor cells" that generate precursors for all myeloid cell lines. The first myeloid progenitor is designated CFU-GEMM for "colony forming unit--granulocyte, erythroid, macrophage and megakaryocyte". In all of these descriptive terms, "colony" refers to a cell that is capable of giving rise to more than 50 cells as measured in 14 day in vitro assays for colony growth, under conditions as set forth in Example 5 of the present specification. These cells will divide at least six times.
The CFU-GM is committed progenitor--in other words, it is committed to differentiating into granulocytes and macrophages only. It is neither capable of differentiating into other types of cells nor is it capable of dedifferentiating into earlier stage progenitor cells. The CFU-GM progenitor cell may then differentiate into a myeloblast. The time required for differentiation from a CFU-GEMM to a myeloblast is believed to be about 1-4 days. A myeloblast is the first of the series of cells that may be referred to as "precursors" to the neutrophils, as such cells, once allowed to fully develop (differentiate), can only form neutrophils, which are only capable of undergoing fewer than six cell divisions and, therefore, do not form colonies in in vitro colony assays as described previously.
Once differentiation has progressed to the myeloblast stage, the myeloblasts undergo terminal differentiation into promyelocytes, which, in turn, differentiate into myelocyte over a course of about 4-6 days. Within another 5 days or so, myelocyte differentiate into metamyelocytes, which in turn, differentiate into banded neutrophils. These banded neutrophils finally differentiate into mature, segmented neutrophils, which have a half-life of about 0.3 to 2 days. The term "progenitor" will be used to refer to stem cells, and cells which can form colonies. "Precursor" will be used to refer to myeloblasts, promyelocytes and myelocyte an, in some instances, metamyelocytes and banded neutrophils, also.
During this progressive, morphologic differentiation, changes in the surface antigens of these cells can be observed. For example, stem cells, CFU-GEMM and CFU-GM are CD34+. Hematopoietic cells that differentiate beyond the CFU-GM state are no longer CD34+. Similar progressions of expression are observed for the cell-surface antigens CD33 and CD45RA. All neutrophil precursor cells subsequent to the promyelocyte precursor cells may be characterized as CD34-, CD33+, CD38+, CD13+, CD45RA-, and CD15+. More mature cells also may be characterized as CD11b+ and CD16+. (Terstappen et al. Leukemia 4:657, 1990). It should be appreciated, however, that such transitions in cell-surface antigen expression are gradual, rather than abrupt, wherein some cells of a particular precursor cell type may be positive and other cells of the same type may be negative for a particular cell-surface antigen. Furthermore, the determination that a particular cell type is positive or negative for a particular cell-surface antigen will depend, in part, upon the particular method used to make that determination. The characterization of cell differentiation by cell-surface antigen expression may be confirmed by other means of characterizing cell differentiation, such as cell morphology.
Specific growth factors react with specific receptors on stem cells to direct their differentiation into committed progenitor cells. These actors regulate the proliferation and differentiation of hematopoietic cells. At least four colony-stimulating factors (CSFs) are known to cooperate in the regulation of neutrophil production. These four factors, which are referred to as GM-SCF (granulocyte and macrophage), IL-3 (interleukin-3), G-CSF (granulocyte), and M-CSF (macrophage), which is also known as CSF-1, are synthesized by macrophages, T cells, endothelial cells and other types of cells, The potential of a progenitor cell to respond to a CSF is determined in part, by the presence of receptors on the surface of the cell for that particular CSF and, in part by the concentration of the particular CSF. There also is some indication for indirect stimulation, whether via an accessory cell or by synergistic action with other obligatory growth factors, such as c-kit ligand, IL-6 (interleukin-6), IL-11 (interleukin-11), IL-4 (interleukin-4), and IL-1 (interleukin-1).
In addition to changes in morphology and cell-surface antigen expression, as neutrophil precursor cells differentiate, they lose their capacity to proliferate. In general, the less mature neutrophil precursor cells, namely the myeloblasts, promyelocytes, and myelocyte, retain their ability to proliferate. However, the more mature neutrophils, namely the metamyelocytes and the banded neutrophils, lose their capacity to proliferate, although they continue to differentiate into mature, segmented neutrophils.
Several methods of treatment have been proposed to combat HDC-induced neutropenia. These methods can partially ameliorate the neutropenia but cannot eliminate it completely. Bone marrow cells alone have been used to provide the cellular component necessary for neutrophil recovery. However, this particular method of treatment only reduces the period of neutropenia to about 2-3 weeks.
Several problems are associated with the use of bone marrow to reconstitute a compromised hematopoietic system. First, the number of stem cells in bone marrow is very limited. Stem and progenitor cells make up a very small percentage of the enucleated cells in the bone marrow, spleen, and blood. About ten times fewer of these cells are present in the spleen relative to the bone marrow, with even less present in the adult blood. As an example, approximately one in one thousand nucleated bone marrow cells is a progenitor cell; stem cells occur at a lower frequency. Secondly, a significant period of time is necessary for a stem cell to differentiate to a mature neutrophil, on the order of at least 10-15 days.
Bone marrow gathered from a different (allogeneic) matched donor has been used to provide the bone marrow for transplant. Unfortunately, Graft Versus Host Disease (GVHD) and graft rejection limits bone marrow transplantation even in recipients with HLA-matched sibling donors. Approximately half of the allogeneic bone marrow transplantation recipients develop GVHD. Current therapy for GVHD is imperfect and the disease can be disfiguring and/or lethal. Thus, risk of GVHD restricts the use of bone marrow transplantation to patients with otherwise fatal disease, such as malignancies, severe aplastic anemia, thalassemia, and congenital immunodeficiency states. About 7,000 of the 15,000 bone marrow transplantations performed each year are allogeneic. Many other patients have diseases that might be treated by marrow cell transplantation (such as sickle cell anemia) if GVHD or graft rejection were not such serious risks.
An alternative to allogeneic bone marrow transplants is autologous bone marrow transplants. In autologous bone marrow transplants, some of the patient's own bone marrow is harvested prior to treatment, such as HDC, and is transplanted back into the patient afterwards. Such a method eliminates the risk of GVHD. However, autologous bone marrow transplants still present many of the same problems presented by allogeneic bone marrow transplants in terms of the limited number of stem cells presenting the bone marrow and the amount of time required for a stem cell to differentiate to a mature neutrophil. In addition, autologous marrow also may be contaminated with tumor cells.
One approach to overcome the problems with bone marrow transplants has been the attempted isolation of stem cells form donated bone marrow, or other sources, and the use of such stem cells to regenerate the immune system, such as after HDC. The theory behind this approach in the allogeneic setting is that the stem cell is naive in nature (has not developed significant host-specific characteristics) and, therefore, will not be recognized in the transplant recipient as a foreign body or antigen, thus hopefully improving acceptance. Furthermore, since these isolated cells contain minimal numbers of T-cells, it may be possible to avoid adverse reactions, as in GVHD.
Problems are also associated with this approach. Since the number of stem cells in bone marrow is very limited and at least about 10-15 days is required for stem cells to differentiate into mature neutrophils, significant in vitro multiplication of the cells must take place in order to generate an adequate number of neutrophils for introduction into the patient. Thus, the transplantation of stem cells at best results in an immunocompromised patient continuing to be immunocompromised for a significant period of time.
Hematopoietic growth factors, such as G-CSF or GM-CSF, have been administered alone or combination with autologous or allogenic transplants of stem cell populations subsequent to HDC. Although neutrophils increase in number as a result of the treatment, the period of severe neutropenia is only reduced to about ten days. Since the production of neutrophils form stem cells normally takes about 10-15 days, stimulation of progenitor cell production and differentiation by hematopoietic growth factors and the eventual reconstitution of mature leukocytes, including mature neutrophils, requires a significant period of time.
Peripheral blood stem cells (PBSC), which have been mobilized with chemotherapy or growth factors, also have been used to create neutropenia. It is believed that the mobilized PBSC represent a mixture of progenitor cells and, perhaps, precursor cells that occur naturally during the recovery of myelosuppressed bone marrow. Again, such a mixture of progenitor and, perhaps, precursor cells only reduces neutropenia to about nine days. Furthermore, the precursor cells in these mixtures probably would not survive freezing, since cells containing granules do not freeze well using presently known methods, and, therefore, could not be stored for subsequent treatments.
Generally speaking, none of these methods is successful in reducing the period of severe neutropenia below about 8-10 days. Such a lengthy period of neutropenia still renders the patient susceptible to infections, the treatment of which requires hospitalization at a significant cost.
Transfusions of mature neutrophils also have been attempted as a means of addressing neutropenia. Such transfusions can be very expensive and involve healthy donors in a procedure that is time consuming, uncomfortable, and risky (Clift et al, Symposium on Infectious Complications of Neoplastic Disease (Part II), Vol. 76:631-636 (1984)). A major concern in the use of mature neutrophil transfusion is that, if transfused mature neutrophils are unable to function and circulate normally in the recipient individual, toxic reactions may result with adverse consequences (Wright, The American Journal of Medicine, Vol. 76 637:644 (1984)).
U.S. Pat. No. 4,714,680 describes a suspension of human lympho-hematopoietic stem cells substantially free of mature lymphoid and myeloid cells but which may further comprise colony forming cells. Such a composition could be used in the treatment of neutropenia, however, given the fact that the production of neutrophils from stem cells requires about 10-15 days, such a composition would not reduce the period of neutropenia.
U.S. Pat. No. 5,004,681 relates to hematopoietic stem and progenitor cells of neonatal or fetal blood that are cryopreserved, and the therapeutic uses of such stem and progenitor cells upon thawing. In particular, the invention relates to the therapeutic use of fetal or neonatal stem cells for hematopoietic (or immune) reconstitution.
U.S. Pat. No. 5,061,620 describes a method to obtain a cellular composition of human hematopoietic stem cells, with fewer than 5% of lineage-committed cells. Such a composition also could be used in the treatment of neutropenia but, once again, such a composition would not reduce the period of neutropenia, since 10-15 days would be required for neutrophils to differentiate from stem cells and CFU-GEMM.
U.S. Pat. No. 5,087,570 relates to concentrated hematopoietic stem cell compositions that are substantially free of differentiated or committed hematopoietic cells. The cells are obtained by substraction of cells having certain particular markers and selection of cells having other particular markers. The resulting compositions may be used to provide for individual or groups of hematopoietic lineages to reconstitute stem cells of the host, and to identify and assay for a variety of hematopoietic growth factors.
There remains a need for an effective means of treatment to significantly reduce, if not completely eliminate, the period of neutropenia. Such a treatment would enable a patient, who has undergone HDC or some other form of chemotherapy, such as that associated with conventional oncology therapy, to combat infection, thereby reducing, if not completely eliminating, the risks of morbidity and death. Similar benefits would be realized for patients suffering from drug, toxin, radiation or disease-induced neutropenia or genetic/congenital neutropenia. In addition, such a treatment also could be used to treat a patient who, although not suffering form severe neutropenia, has a reduced level of neutrophils.